I'm very familiar with these, both from a service provider and enterprise perspective. Most of the time it's a 3rd party company fishing for business (they get a commission for any license shortfall/penalties).
If it's a 3rd party rather than MS themselves, and you haven't signed an agreement with MS which allows them to audit you, you can normally just fob them off with "We run Office 365 exclusively, and only license our computers with the manufacturer supplied Windows licensing." Sometimes they will ask for a sample of your OEM keys to make sure, but they normally only ask for 5.
If you have signed an EA with them, then you are obliged to comply. They can be delayed for a bit, but it's better just to get compliant and fill in the spreadsheets. You can also tell them you cannot install their MAP tool due to company policy, and just do the audit manually.