Securmate showed their concept pocket-sized & portable premise security system at the CES 2016 Eureka Park startup pavilion.  At a minimum, you have a control / communication unit about the size of a medium smart phone and a matchbox-sized sensor (2 AAA batteries) which communicate via WiFi.

You can secure the door(s) or window(s) of a hotel room, a motor home, a dorm room, an apartment or basement storage unit – pretty much any open / close access panel with one or multiple sensors. (no hard-coded limit!)  When a security breach is detected, the sensor signals the control unit which then can text ($3/mo) or email the designated entity.  www.securmate.com