Newegg has the Rosewill RSHS-11002 Travel Alarm for $15 - $8 off with coupon code EMCYTZT1142 [Exp 2/5] = $7 with free shipping. Hang on the door knob and insert the door clip sensor into the door gap to create and alarm, when tripped will sound 100 dB high pitch alarm.
May want to check if this is legal 1st. May get you in trouble in a hotel.
For sure would freak the cleaning maid.
I bet Mahmoud al-Mabhouh really wishes he would have had one of these deals.
^
That's good stuff to ponder. In that situation, who cares about the law.
Wouldn't have mattered anyway, There were 4 Mossad agents (allegedly) already in the room when he returned.