The Gardai (Irish Police) warned recently that a gang were operating on the Irish South west Coast stealing outboards in quantity off unattended boats from almost anywhere including on moorings. Apparently they slip the moorings and get the boats aground and remove the engines for sale.
Official advice is to remove engines from boats when not in use
But do try watch where you're leaving your boat - you may be safer on a mooring than round the back of a quiet house or yard, ensure you have the engine number and check your insurance is paid up and that you're compliant with the small print. For example mine won't cover the engine unless it's locked to the boat and they've been given the serial number in advance.