@ mrdynamo
Ur case seems somewhat complex (minor) or in other words some what different from the usual cases! But nothing to worry. If the stay is genuine in another country then there would be no problem, I bet.
Yes, they do confirm from all those counties where one have had ever lived. Just read the declaration page(4) given in the Schedule 1 Background/declaration form, IMM008_1e, this is clearly given there. They just investigate it for security purposes. Regarding "what will they do check' I am not much sure, but can guess that the security checks would include thorough investigation about records such as addresses, character, criminal record, relationships , siblings etc etc.
Best wishes