Yes, most insurance companies are very vague about the above. Is it just to deter people from doing this? Funnily enough, on my own policy, in the Product Document, it states that I have Comprehensive cover when I drive other cars , providing the car in question is not owned by me and is 3,000CC or lower.
Current:
'06 530d Silver Grey, '04 325Ci Orient Blau
Previous:
'03 M3 Silver Grey
'02 325Ci Imola Red II
'98 318iS Cosmos Schwartz