Usb says it's using data, even though it has been formatted and completely wiped out clean? Can this be malware?

A friend of mine borrowed a Usb of mine, the problem is his pc is completely full of malware (he says so himself, he has had his pc for about 7 years and never used any kind of anti-virus or anti-malware, even though he constantly downloads shady stuff).

After he gave it back I quickly formatted it but strangely enough when going to "Properties" it always shows it's using X amount of data which also strangely enough it also varies from 60MiB to 95Mib, I also deleted all partitions using GParted on linux (It seems he was using the usb to install a linux distro, so I deleted the extra partition), and I ran both bitdefender and malwarebytes on windows but they both say the usb is clean, so my question is, can this really be malware? Or is it something else? Any ideas?

Thank you all in advance!

Edit: Is there also any way to know what those Mb are? Since the usb itself seems to be empty, I've even checked for hidden files, alas to no avail.

Edit2: The Usb is a 32gb Kingston Micro.