Do you do it with the engine running? You will get a check engine light if your gas cap is loose/removed...and if you fuel it with the engine running, that would most likely trip a check engine light. The only other thing I can think of is maybe the can allows more air to enter the fuel system than normal. If the light goes off after a bit and isn't kicking on randomly, I'd just ignore it.
If it starts coming on at random frequently...change your gas cap. If that doesn't fix it, you'll prolly need your mechanic to do a "smoke test" on the fuel system.