A common scam is when a host family email an au pair accepting their services and sending them a (false) cheque for their travel. A little while later, they cancel the agreement with a plausible excuse and ask the au-pair to return the money by wire, often Western Union.

100% correct. It can also be:
(a) Buying an item off Craigslist/Kijiji, the new home of scammers.
(b) A fake job offer hiring you as a 'mystery shopper'
Just substitute the words, same scam. Please remember folks that even in the 21st Century it can take 4-6 weeks for a check to completely clear your bank. The funds seem available after 1-3 days, but give it time.