Well it's true that, by default windows doesn't log activities such as modifiying file.
But I think using Search you can have some clues.
1> Go to search, select the hard drivies in which you suspect such activity has taken place.
2> Click on "Search Options" > then click on "Date" now you can enter the date as you desired.
3> If you cna't set "Time" then search by only date.
4> Make sure that you are in detailed view. If not then go to following menu View > Detailed and
5> Now arrange the results by either "Date Accessed" or modified you can do this by View > "Choose Details ..." here you have to check "Date" "date modified" and so on for each one with date.
6> Now leave "Search for .. " text box blank & click on search.
7> Now you will see all the files modified or accessed in the date you specified
8> Now go to View > Arrange Icons by > And select whichever one you feel more relevent like "Date Accessed"
9> Now along side the date you should also notice that there is time also
10> Now this time is what you require to perform crosscheck. Note the date down & crosscheck that who Accessed the file on that time. If you can't find anyone doing that then you have reason to suspect.
Hope you find it helpful