From a couple posts back, it looks like a flashlight but I have heard some airlines are handing out chem detectors that look like a flashlight to be used during the walk around when picking up an aircraft from the inbound crew. Since chem is supposed to be off in descent, any residual means procedures weren't followed. I doubt anyone would turn a guy in though, that wouldnt go over very well. Might be bs, just what I heard recently and it would help to explain why a guy would do a walk around in the daytime with a "flashlight." Good catch, but most regular types would never notice.