People who are having an affair with a co-worker often think they're being discreet, but others can pick up on the body language and tension between them.
People who are disinterested in a conversation or activity may try to hide it, but others can often sense the lack of engagement in their body language and facial expressions.
People who surf the internet at work may think they're being sneaky, but others can often tell by the way they're looking at their computer screen or how quickly they switch tabs.