Some transatlantic security people in Heathrow, including those from AA, like to stick a sticker on your passport to indicate they have asked you random and fairly pointless questions about your travel, work, and place of residence. They get quite upset if you remove the sticker and put it on anything else before you get to the boarding gate.... so I remove it shortly after boarding. The adhesive is far less tacky if removed quickly than if left for a long time, so the best way to deal with them is to clear your passport of them as soon as possible. Passport covers do not avail you.