At first, I thought my T was blank slate, though it turned out he just really held back until he knew me.

I think self-disclosure is a pretty difficult topic. It probably depends a lot on the client what can be shared and what can't. For example, my T once shared something of his own life that was completely connected to what we were talking about and was really just showing he knows the feeling and situation and can empathize. But it made me go off the rails real bad. It's probably difficult to tell in every situation whether sharing is good or not.

Regarding the where you go on vacation: I think it's an innocent enough topic that probably in most cases won't hurt the client and could be shared. My T this time shared where he was going (staying at home), but I didn't ask and never have, he only shared due to things going on with me.

I can, however, kind of see why in certain situations the T would not share it. First, of course, if the client would have a high probability of showing up at the place, if it's somehow easy to tell where T will be exactly. This is probably not the reason most of the time. The other thing I can think of is this: it might make the client feel better in the short run, you know where T is, you can think of the place, imagine him there, if something happens in the world, you know he's not affected... I think this is a false sense of security and control. Knowing where somebody is does not in any way affect the probability that they get hurt. Of course there's activities and places in the world that are more dangerous, but I think it's safe to assume that usually that's not what Ts do in their time off.

So, we really have two options if something happens in the world: First, T is not there. In this situation, the client will be calmed by his knowledge of where T is. While this is certainly nice, it also takes away a great learning opportunity. If the client would not know where T is, he would have to calm himself and use self-soothing, without contact immediately available. In my mind, these opportunities are far and few between. So it's a good exercise field.

Second, T is in that location. In this case, the client will freak out. However, even if we look at big disasters, usually the probability to be hurt is actually still quite small. Humans tend to not be very good at estimating correct probabilities.
Let's make a quick example: at the Boston marathon bombings, 3 people died and 281 got hurt. There were about 25'000 runners and it is estimated that each runner brings three people with them, that means another 75'000 people watching. So let's assume there's 100'000 people, although this is probably too low (there will be people watching that the runners did not bring along). This means the probability to get hurt during those attack was 281/100'000=0.281% . For comparison, getting cancer if you're about 40-50 years old is 0.35% and if you're over 60 (which my T falls into), it's 1%. That means my T is 4 times more likely to get cancer than to get hurt in that attack, let alone die.
And that is only if T actually decided to go to that marathon. However, if the client knew T is in Boston, it does to them not matter whether T isn't even into sports or what the probabilities are. It will be a big event of worry and fear. Yes, it probably sucks to be in a city where something like that happens, but it's really not that probable that there's lasting damage from it.

This doesn't even take into account that the Ts plans might have changed, planes might be delayed, not everything about trips is shared and so on.
Finally, I think this is sadly just a part of life. We do not know where our loved ones are at all times and we can't control whether they get hurt. Knowing where they are does not change this. So, I think not knowing is a great way to practice being okay with this, while telling the client where you go will take away that opportunity.