Older Mails that have been encrpyted with S/MIME before GpgOL was installed don't get the updated Lock Symbol, when S/MIME is activated in GpgOL.
As you can see in the screenshot this account has some older S/MIME encrypted mails in his sent folder and some newer encrypted ones. The ones I sent with the freshly installed GpgOL have the correct Lock Symbol next to it, the ones that were already there don't have the correct lock symbol.
It was tested with gpg4win 3.0.1 on Windows 10 64bit with Outlook 2010 64bit.