As a regular design element for static locations that are likely to be dungeon adventures, I like to have some place near the beginning (or any opening) that allows for some Game Master flexibility. It's nice to set up such adventures with a way for the GM to account for a range of player character levels, and a secret room near a main entrance creates just the space for such things. Should it have remained secret? Why not? If I say so. And if so, then I can put whatever I think the PC group might be lacking to keep the adventure interesting, perilous but not impossible. Are some of the PCs under-armored or under-armed, a guardroom is the perfect place to have some spare chainmail and hammers and axes left around, in keeping with the Dwarven theme. Perhaps there is a cabinet with some grappling hooks, spikes, or other general supplies, though perishables are out of the question, so no foodstuffs or rations.
Anything the GM decides might be useful, and perhaps a few red herrings or clues to what lies within, can be proffered early in the adventure by this means. If this isn't needed, a GM can include a denizen in this room along with some treasure. The secret door can be randomly found in this case but if the room is used to even the odds a bit for the PCs, the "secret" door can be left noticeably ajar. We'll leave this room at that for the time being and after we flesh out the area more fully, maybe we can circle back and add more details including some narrative text.