As they say, every day is a school day … todays lesson was learnt while trawling Exchange 2003 Servers and finding a while load of .ifs file in the database folder.
So a bit of googling and I discovered this!?!
Files ending in .ifs may appear in folder containing database log files
During the operation of Microsoft Exchange Server 2003 or Microsoft Exchange 2000 Server, you may notice several files that end in .ifs in the Mdbdata folder (or wherever you have the database files stored).
These .ifs files are temporary files that are used during the normal operation of the Information Store service to cache directory listings and Exchange Installable File System (IFS) Master File Table (MFT) listings. The .ifs files are created with the FILE_FLAG_DELETE_ON_CLOSE flag set. The Information store process automatically deletes these files, and if any .ifs files are accidentally left behind (if the server stops responding, for example), they should all be deleted when the Information Store service starts. It is dangerous to delete .ifs files manually when Information Store service is running, but you can delete them safely when that service is not running.