If I understood right T and Mary Smith share a home address and Mary sometimes brings T to work. I think the likeliest and most obvious explanation is that they're a couple. Sure there are other possibilities but do you often get magazine subscriptions in your name sent your colleague's home address? They could be roommates, definitely, but how commonly do you meet people over 30 or 40 living with roommates compared to people in the same demographic living with partners or spouses? Way less, right? Also in my experience it is very, very rare for straight women to avoid making casual mention of their husbands (if you've been seeing her for awhile) whereas it's quite common among gays and lesbians to refrain from mentioning their partner/spouse at work.