Walking around in the past, as a thief, probably without detect invis, in the teens, is like asking to get ganged.

1. Ashes and FoN are high-traffic ranking areas, meaning you're likely to encounter ranking groups all of whose members are in your PK.
2. Even if you don't encounter any ranking groups, you're likely to encounter PK-hungry types who've come to the past to LOOK for the aforementioned ranking groups.
3. You're a low-level thief, meaning you have low hp, low damage output and probably no shield block. You don't present much of a deterrent.
4. If you don't have detect invis up then most PK savvy people will be able to get the drop on you. And not just the mages.