I have been doing muay thaï for 4 years. Once per week we do sparring as a part of the training. The sparring is relatively light, the goal is not to knockout the partner, not even to cause injury actually. However, some hits can pass and the head can bounce a bit. I do not even get dizzy after these types of hits. I do not compete. Under these circumstances, is there any danger of having a brain trauma or concussion?
Yes, there's a danger of concussion. My answer to another question on concusssions.
Do you know what sports end up getting a surprising amount of concussions? Soccer and volleyball. It's not because these folks are getting hit in the head all the time - it's just that when they do get hit, they have had little conditioning to help deal with the blow.
Any hit to the head has potential for concussion, and the problem of concussions is that damage builds up over your lifetime. You should see a doctor or sports doctor to get an assessment at some point. There's now pretty good tests using computer apps to measure your cognitive ability against your own baseline to see if you're suffering light concussions and an impairment over time.
Some people are more susceptible to concussion than others for no apparent reason. You may be one of those folks who catch them easily, and the only best advice is to reduce head hits as much as possible, which may mean changing martial arts or your sparring rules drastically.
Since you don't compete, and you're sparring once a week, you probably aren't taking enough damage to cause noticeable long term affects. One issue is the shots don't always have to hurt to cause brain damage.
Any "bouncing" of the head causes damage. Jet ski riders and slalom skiers have been shown to have some of the worst brain damage of all athletes, due to that bounce (google search provides some great sources on the research). Is your sparring comparable to that? Probably not, but it's worth noting. If you ever get a headache from a couple hard shots, take some time off. Go see a doctor if the headaches won't go away.
I know you already said you don't go hard to the head, but sometimes guys get feisty. How we spar is: light to the head, hard as you want to the legs and body. It allows for people to rip some great shots. You can drop guys with a nice leg kick or body shot, and yet they're perfectly fine after the initial pain. Maybe look into this style of sparring if you're feeling spicy.
In short, yes. There's always that danger.
When you're training with experienced partners, this danger is near to none, as they know how to measure hit strength. However, if you train with beginners, even if it's light hits, they don't know how to measure hit strength yet, and may actually hit quite hard without intending to.
When training with only experienced partners, accidents can still happen. A slip putting extra weight in a kick, a reflex kicking in, ... . These things can happen, no matter the experience. Experience just lowers the chances of accidental hard hits.
Thirdly, given enough time, even a mountain can be removed with a shovel, one step at a time. Get enough light blows to your head, and it will definitely still leave a mark. It will be less bad than a few heavy hits, but it will still leave a mark.
And last but not least, "relavitely light" is... well... relative. Now I'm not saying lightweights pull their punches, but a lightweight's punch is light to a heavyweight. If a heavyweight does a punch that's light to heavyweights, it may still hit harder than a bantamweight fighter's ever been hit. Even within bantam / feather / lightweight divisions, there've been plenty of deaths related to brain damage (eg. Davey Moore in 1963, Johnny Owen in 1980, Cleveland Denny in 1980, etc.)