Tritium is generally very very low in the environment in general and 55x "Above average" is still much lower than you would ever expect as an indicator of fallout from any fusion weapon. Remember that radioisotopes can be measured at extremely minute levels, so if the general amount of tritium is very nearly zero than 55x is still a tiny tiny tiny itty bitty amount. The fact that they don't give an absolute amount of tritium (only "55X") is probably because it would look a lot less impressive if expressed in parts per billion or even parts per trillion...
... Now as far as what could account for the increased tritium. Even before I read the report I immediately thought "radiolumonescent devices" because thats the most common use for tritium and would more than account for the amount found. Basically these are self-powered glowing items which glow brightly and continuously in the dark, even without the need for "charging" with light. The amount of tritium in the, in terms of curies is quite high. They are common in emergency signs in aircraft. I actually think the amount of 34ci on the aircraft is a tad bit lower than I would have thought.
Also, as mentioned, in the sites of weapons, tritium is commonly used. It's found in higher-end glowing watches, but I doubt that would be anything signifficant. I have an exit sign which is labeled as containing 80 Ci of tritium as of manufacture. That's actually a lot. Most emergency exit signs have 20-40 or something like that.
But that having been said, they are quite common for emergency exit signs, especially because they can be installed and meet qualifications for emergency lighting without the need for any wiring or drilling of holes etc..
I do not know of any in the WTC or surrounding buildings, but I would not be surprised if there were. You would not need many to account for these levels. Anyone can buy these things. They're common. They have them in the library in my town and the walmart. They range from $150 up, but that can still be cheaper than a standard battery-backed exit sign and an electrician to wire for it. So, if you look around you see them a lot.
The world trade center was a HUGE office complex and since you don't need to have any sort of special licence for one of these, there may have been some.
Add in the weapons sites. The tritium signs on the aircraft. Possible broken signs in nearby buildings. It can more than account for the modest increase.
