As AnalogKid mentioned, a nockoff arduino micro and maybe a small ultrasonic sensor would be the least possible parts, and probably the easiest.Panasonic makes pretty small PIR sensors. After that, uC (microcontroller) will be the smallest, cheapest, and lowest power timer/logic circuit - IF you already know how to write code and program one. Of course it can be done with discrete logic chips, but even with sufrace mount parts it will be larger than a uC solution.
There probably are small motion sensor modules on ebay, maybe as an Arduino shield.
Ah gotcha.I was think more like an 8-pin PIC as the sensor interface, timer, LED driver - everything. For a range limited to 18", a better sensor might be ultrasonic. However, the low-cost modules on ebay are larger than a simple PIR sensor. Don't know about how hungry they are.