Any hotwire Maglite would be a good start... They would show the most bang for buck as a sleeper, but if you want something even more subtle, then you need something like an iTP A3 Eos... looks like a simple keychain light but puts out about 8 times more light compared to something you would pick up at a supermarket.
There are literally dozens of lights that fall into this category, depends on how stock you need it to appear, and what level of consumer you are aiming at.
For instance, if it was your typical Australian who still thinks Incan mags for $100 are the best light in the world, bar none! then a simple SF 6P incan will really impress, but if you are looking at someone who is aware of high efficiency LED's and maybe even owns something in the 100-200 Lumen category, then you will have to start looking at dropins or mods... for instance, I have an SST-90 in a 6P (600+ Lumens... possibly a lot brighter at turn on with LiFePO4 batteries, maybe upwards of 900+) which (for a short period of time) kicks the crap out of all my lights but looks like a standard light