Namely - captain Wacław Radziszewski (commander of IV battalion 82 IR - which did not leave the fortress together with other units, as you remember) - was murdered by NKVD in Kozielsk in 1940.
So he must have been captured by the Soviets !
I have just found more detailed info about it - they wrote about it in this Polish forum:
http://www.dws.org.pl/viewtopic.php?f=7 ... &sk=t&sd=a
IV battalion of 82 IR was ordered to protect the Polish withdrawal together with 2 sapper company of podporucznik Kazimierz Giaro, but dispatch rider sent to Radziszewski did not come back (anyway - he received orders - but did not want to complete them - dispatch rider was probably missing during the way back to Giaro).
During the night on 17th of September (but few hours after the Polish withdrawal), part of Polish battalion - with one 75mm gun - left the Citadel (part of the Citadel which was called "kobrynskie fortifications") and occupied fort "Graf Berg" (then it was called "Fort Sikorskiego", nowadays it is "Miasokombinat").
According to other sources it was not Fort "Graf Berg" but Fort V (located south from the Citadel).
Allegedly as long as not until 19.09.1939 the Germans noticed that there were still Poles inside the fortress.
Since that moment, after rejecting capitulation offert, several days long battle for the fort "Graf Berg" (or "Fort V"
Between 19th and 20th of September Polish battalion was repulsing German attacks.
Between 21st and 26th of September Polish battalion was repulsing Soviet assaults.
Only on 22nd of September Polish defenders destroyed 1 Soviet AFV and repulsed 3 infantry assaults.
On 24th of September and on 25th of September a lot of Soviet infantry assaults from all sides were repulsed.
On 26th of September Soviet forces introduced heavy artillery into action - under this condition further Polish defence became hopeless.
During the night from 26th to 27th of September rests of the battalion left the fort and marched towards the village called Murawiec.
In Murawiec Polish battalion was dissolved.
Captain Radziszewski changed his clothes for civilian and came back to the city of Brest - to his family.
But the Soviets soon discovered who is in Brest and NKVD arrested him.
He was murdered in 1940 in Kozielsk as one of victims of the Katyn murder - his surname and name can be found in the list of Katyn murder identified victims, as well as names and surnames of several other officers who took part in the defence of Brest in 1939:
Two officers from the same battalion:
porucznik Aleksander Gradziuk - commander of 1. company of IV battalion / 82 IR.
podporucznik Witold Skwierczyński - commander of 2. company of IV battalion / 82 IR.
And - surprisingly - also 2 officers from 56 sapper battalion, 2 officers from marching (spare) battalion of 34 IR and one officer from marching battalion of 35 IR - and also general Kazimierz Plisowski.