Espionage at the Chinese Restaurant

Question

The Republic of Lateralia had been beset on all sides by hostile nation-states, each of which were eager to diminish the Republic's sovereignty. Lateralia's behemoth of an economy was an object of envy, and some ambitious kings and war-princes felt that the only way to become competitive would be to embark upon a campaign of subterfuge and sabotage.

Another thing that rival states couldn't stand about Lateralia was its plethora of high quality Chinese restaurants. Every city of appreciable size had a variety of take-out restaurants, all-you-can-eat buffets, and posh teahouses featuring a variety of food and drink based upon Asian cuisine. Agents in the employ of Lateralia's secret service had tailed two suspected enemy spies, codenamed Sherlock and Hercule, to a particular town. It was suspected at the time, and later confirmed, that Sherlock was somehow clandestinely communicating sensitive information about Lateralia's defenses to Hercule, but at the time, no evidence could be obtained to prove how it was done.

On August 29, Sherlock was observed dining at The Happy, Oh, So Happy Chinese Buffet. The eatery was known to be frequented by people suspected of anti-Lateral activities, and thus the entire establishment, with the exception of the bathrooms, were equipped with monitoring equipment such that authorized agents could record all of a target person's activity. Consequently, Lateral counterspies reviewed all the video and audio information, but unfortunately they were unable to find anything of particular interest, other than the fact that Sherlock ate quite a bit of broccoli and spent a fair amount of time in the bathroom. On August 30, the police arrived with a search warrant. Between the findings of their search and the sifting through of the trash from the previous day conducted by Lateral spies, no unusual physical objects were discovered as being left behind by Sherlock. On August 31, Hercule dined at the same eatery, and again his movements were carefully studied. It was noted that Hercule seemed to have a particular fondness for the doughnuts, and he also spent a fair amount of time in the same bathroom as Sherlock, but nothing could be determined about unusual behavior. Out of desperation, agents even detained and searched Hercule as he was about to drive away, but no papers or other communicatory instruments could be found on his person or in his vehicle. Nonetheless, Sherlock had successfully communicated the address and apartment number of a safehouse used by anti-Lateral spies.

The restaurant was generally restaurant-like, with only a few features of interest. There were stacks of plates, tables laden with food patrons could help themselves to by adding food to their plates with tongs, and mirrors along most of the walls. It was known that there was nothing hidden behind the mirrors other than the equipment Lateralia itself had installed for security purposes. There were eight individual bathrooms, four designated for males and four designated for females. Each bathroom communicated to the same hallway via a door with a padlock, which displayed an "occupied" message when locked and a "vacant" message when unlocked. Within each bathroom there were a sink with hot and cold water handles, a soap dispenser, a paper towel dispenser, a trash can, a mirror, a ceiling-mounted light, a motion-activated light switch, a toilet, a toilet tissue holder with four rolls of toilet tissue, a shower, and, for some strange reason, a dining chair. The four bathrooms for males also included a urinal "to help save water" (according to the owner). Also, each bathroom had a poster which read, "Employees must wash hands after using the bathroom. It would be appreciated if customers did so, too. -Management"

How was Sherlock able to communicate with Hercule without alerting the Lateral authorities?

Edit: For extra clarification, the following assumptions are reasonable:

• The hallway that communicates with the bathrooms is not considered part of the bathrooms, and thus is under the same surveillance as the rest of the restaurant. Only the bathrooms proper are excluded.
• The foreign spies visited one and only one bathroom during their visits. This happened to be the same one.
• There were no secret compartments of any kind where papers or other conspicuous physical objects could be hidden.
• There is some degree of maintenance and cleaning performed in the restaurant and each bathroom at the end of each day. Floors are swept and mopped; empty soap containers, paper towel dispensers, and toilet tissue rolls are filled or replaced.
• Anything "left behind" by Sherlock would not be immediately obvious to the police or secret service under the conditions during which the restaurant was searched.
• The address is short enough that it can be assumed it is easy to be memorized, but Hercule would only be able to learn it in the first place from his experience at the restaurant.

Answer 1

Could Sherlock have:

left a hidden message in the mirror using the soap. He writes the message using soap and cleans it up afterwards carefully. Also this doesn't have to be just the mirror it could have been the toilet, the urinal, and maybe even the poster

So when Hercule went in there

He just turned on the shower and waited for the steam to fog the mirror causing the message to show up

Answer 2

Here is one possible solution. Like any such solution, it is dependent on how thoroughly the SS searched the premises, and how good they are at recognizing hidden information, as well as how thorough the cleaning crew is.

The information was hidden behind the "Please wash hands" poster. Since this is a "high quality" chinese restaurant, we assume the cleaning crew is pretty thorough, and does things like wiping down the walls and mirror. However, it is unlikely that they would remove the poster. Now, lets assume that one of the broccoli offerings at the restaurant includes some kind of slightly sticky sauce - perhaps one containing some orange or lemon. Even a little bit of sugar would create a slight sticky spot on the wall. So Hercule need only grab a couple powdered sugar donuts, blow the sugar onto the wall, and see where it sticks.

Answer 3

Partial answer:

Broccoli contains phosphorus. Phosphorus glows in UV light (black light). He entered the bathroom with a mouthful of broccoli and wrote the address with it on the mirror. Now how do you make a black light out of doughnuts? No clue.