happen to …

Happen can be used with a following infinitive to suggest that something happens unexpectedly or by chance.

• If you happen to see Imogen, ask her to phone me.
• One day I happened to get talking to a woman on a train, and she turned out to be a cousin of my mother’s.

In sentences with if or in case, the idea of by chance can be emphasised in British English by using should before happen, but this is no longer very common.

• Let me know if you should happen to need any help.
• I’ll take my swimming things, in case I should happen to find a pool open.