The rules on ysoki cheek pouches are rather vague on whether objects held inside them are actually concealed. It seems possible that someone else would be able to determine that the ysoki was concealing something either by visible bulge or by the impact the object has on the ysoki's speech. Is there any published Starfinder source that addresses this question? If not, is there anything about rodent biology that suggests a good ruling?
When a ysoki is holding items in his cheek pouches, can others tell if they have something in there?
There are no further rules on this
A search of available material suggests that this ability seems to be largely up to GM interpretation, beyond the Bulk limit. There are no further mention of it in the Ysoki material nor any of the search results for cheek pouch(s) on Nethys.
Real-world rodents with cheek storage are very obvious
As you can see in this Youtube Video, even a single significant sized object is quite apparent.
\$\begingroup\$ That video is really something! I'm not sure I would rule that a single L item was noticeable, but certainly a ysoki carrying close to capacity would be noticeable. \$\endgroup\$– ruffdoveOct 20, 2020 at 2:58
\$\begingroup\$ I'd say it depends on the object if I were running the game. A cestus battleglove would be far easier to miss than an acid cannon pistol, for instance. As long as the players understand that their cheeks may get checked by savvy NPC's, it would be pretty difficult to break the game with the ability so I'd lean toward fitting the narrative. \$\endgroup\$– IfusasoOct 20, 2020 at 4:30
From a rules perspective, there's no differentiation between a Ysoki cheek-pouch and any other quick-access pocket, thus it would be a normally-contested sleight-of-hand check to conceal an object of light bulk there. Anything larger would be readily-apparent.