There could be an element of doubt in "Piensan que"
Would you use subjunctive to translate?
They think that the missing jewels might have been stolen but they are not sure.
She might have left them at the hotel.
There is indeed an element of doubt in "piensan que..." making us think that this would trigger the subjunctive; however in our mind (as natives) we think of "Creo que..." and "Pienso que.." more as "declarations", not doubt. This is why these constructions are always taught as kind of exceptions to the rule.
Even if you keep adding elements of doubt to the whole sentence like you did here:
"They think that the missing jewels might have been stolen but they are not sure." There is no subjunctive at all required. This would be:
No subjunctive there at all.
If we change the introductory clause to a negative, this would definitely trigger it:
"They don't think that the missing jewels have been stolen."
"No piensan que las joyas desaparecidas hayan sido robadas."
I hope this helped.
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