If the incident is not your fault and your insurer makes a full recovery from the responsible person, your no claims discount will be unaffected. However if an incident occurs which is not your fault, and they are unable to recover the cost, your no claims discount will be reduced at the renewal of your policy.
How much NCD you will lose depends on your insurer, but typically for each incident settled against you, you will lose 2 years No Claims Bonus. So if you started your policy with 5 years No Claims Bonus and had an accident that was your fault, you would still have 3 years No Claims Bonus at the start of the next policy term.
If the incident is your fault your no claims discount will be reduced. However, your no claims discount may be protected if you have paid extra.
In the event of a theft claim your insurer will probably reduce your no claims discount, as it is highly unlikely they will be able to locate the culprits and recover the costs.