Yep, I can totally see that as a version of events! Because there's no need for Tim to have blown off Sarah, so obviously she was just a cover. "Spending time with Sarah," is clearly a code they use all the time in the office.

"So what are you doing this weekend, McGee?" asks Ziva.
"Oh, just spending time with Sarah," Tim says, not looking at Tony.
Tony makes a mental note to check how many condoms they have left.