The message the boomerang left for me is:
Please let me go!
The first thing to observer ...
... are the top and bottom margins which are made up of the letter pairs SH and OT. The keen puzzler's eye identifies these as Morse code. The OT is the dot, because the word OTOTOTOT can only be an H, which has four dots. There's no English letter that has four dashes. And of course, this correlates with dOT and daSH. Wwe get:
.- - -... .- ... .... / .... . .-.. .--. ... / -- . / ..-. .-.. -.--
-- --- .-. ... . / - .- -.- . ... / -- . / ..- .--. / .-- .... .. .-.. .
It decodes to:
Morse takes me up while Atbash helps me fly.
(The boomerang is thrown up from the ground, so the bottom line come first.)
Now the next step is ...
... to do as we're told. The boomerang itself is made up of letters, its trail is made up of other characters. Apply atbash to the letters of the boomerang. The Atbash cipher reverses the order of letters in the alphabet, so that A is Z and vice versa, B is Y and so on. We get:
\KOVZHV_/O V G /N V-T L ► \PLEASE_/L E T /M E-G O
But that's not all. The boomerang ...
... has also used the trace for encoding a message: The trace is made up of decimal digits and of symbols that can be typed on US keyboards with a digit while holding the shift key. The instruction says to use Morse here. If we treat a number as dot and a symbol as dash, we get:
.-.. . - -- . --. ---
which decodes to Let me go.