Robots getting link

I send emails out to clients, each email has a link to our website and there is unique querystring for each email.

Lately i have noticed a couple of times that someone/something has been going to our website with their link but it also has some extra stuff on the end (path to the icon image we have on the webpage).

I have seen the emails that were sent and they dont have that extra stuff on the end of the querystring.

Would a robot be doing this, if so how do they get this very unique link?