Why would the surviving forsaken not have any on them when freed? The simplest reason is probably that they had to check them at the door. We know that you don't channell near SG without the DO's permission. Secondly if you do before the DO tells you how pissed at you he is there could be unforseen reactions between the OP and the TP. Thirdly we know that the best way to get ahead among the Forsaken is to kill your rivals.

Put those three together and I could easily see a rule that no one is allowed angreal or sa'angreal anywhere near the bore when there are staff meetings being called. Otherwise the meetings could turn into open battles.

So for example Lanfear leaves all her angreal etc at a safe location goes to the meeting and normally will retrieve her gear when she leaves. Instead LTT and the Hundred Companions show up, she ends up sealed in the bore and when she gets out her safe room with all her stuff is buried under the sea.