If someone is keeping a secret, in the US one might say "tell me!" However, I was in the UK and I heard a person say "tell us!" although she did not appear to be with anyone. Am I misunderstanding or is this common?
Yes, it's quite common. It's non standard British English, akin to the "royal "we"".
I use it sometimes myself, though I don't really know why. A couple of examples for you that I can hear myself saying:
When making my way through a crowd of people. (It's just me, though sometimes the "us" form sounds a bit nicer.)
Do us a favour!
Used in indignation similar to "Oh, please(!)". Your example is also one where this is a common trait.
This usage is even more common in Newcastle than the rest of the UK, often making its way into lots of everyday phrases.
Give us a call
Just me. Nobody else.
"Us" for me is common in the north east (of England) particularly Co Durham Land of Prince Bishops.
It's just an old English way of speaking. Many people say "us" but if they are writing will use the word "me". I was born in Sunderland and I use it some times, depends who I am talking to.
If you listen carefully you may notice
- "us" meaning me sounds a bit like "uzzz" and
- "us" meaning you and me sounds like "uss".
That's my family and friends anyway or anyhow as we sometimes say too!:).