Bit of a silly worry but anyway ...just moved into a house through my local H/A
Found out in the last few years a few tenants have lived here.
Sat thinking after a tenant moves out they hand the keys back in.
What is stoping them (if they were crazy ) getting a key cut before they handed them back and potentially letting themselves in your house whenever they want.
I'm sat here rather paranoid now 
Imagine if one was a serial killer