The DW books occasionally refer to secret weapon caches located on planets which can be used by kill-teams.

What would logically be contained in such weapon caches? Assuming they wouldn’t be regularly replenished, how long would stored (bolt)ammo remain useable? How long can you store hand grenades?

And how would a Kill-team be able to access such a cache? Secret codes inprinted on the brain triggering during a challenge & response security test procedure? Space Marine DNA markers?

I'm sure that if a Kill Team were authorized access, secure/coded telemetry would give them the location and access codes to find and open the cache storage unit. Keypad entry would be the most likely form of access. It would only be divulged by the Inquisitor tasking the Kill Team or senior ranking Watch Fortress/Outpost cadre. They would also very likely be in very remote locations too far from an Outpost to get resupplied from.

Gene-coding would be good for recognising 'you are astartes' but, unless the cache is short-term, it's not much more use than that. The Deathwatch has a high turnover of personnel and they have no common gene-strain between them; different chapter geneseeds means no easy recognition patterns unless the cogitator essentially has the entire Jericho Vigil's roster stored in it.

More likely is secure access codes stored in the astartes armour that you can be 'loaded with' at Erioch, or - as noted - a keypad with a numeric or pass-phrase.

Stored ammunition today is X years - varying from weapon type to weapon type, but from a couple of years to a few decades, decreasing depending on unpleasant environmental factors like temperature or humidity (rule of thumb - +10 ^o temperature halves the life). With the high-end technology that the astartes use, however, decades to centuries of storage is perfectly possible if one waves the 'master-crafted mechanicus technology' wand; especially since the storage vault may be conditioned, void-decompressed or even held under a stasis field (in which case caches dating back before the fall of the Jericho sector may be perfectly useable).

As to what you'd store....

My first call would be bolter shells and grenades, as noted. Simply because specialist ammo is situation-specific, either you'd have to stock sizeable amounts of different ammo types, or - more likely - just mag after mag of standard Mars-pattern Mass-Reactives.

Small arms and sidearms would not be unheard of, too. Essentially, I'd probably fit out a small 'emergency' cache containing enough weaponry to outfit a standard astartes tactical squad, sergeant, heavy and special weapon options included. That's enough to handle any conceivable situation a single kill-team should be being asked to fight unsupported.

More than that is getting too much valuable hardware to just bury in a hole in the ground somewhere with no plan for its use.

Larger caches, may, of course, approach watch-fortress armouries in size, but those are going to be much more secure and may not be unmanned - either having a keeper stationed aboard or (it is an armoury, after all) an ancient....