Signed up just to reply to this thread as I’ve run into related situations during the past few years. I know this is last year’s thread, but I am very interested in how this ended up turning out for the OP.
Where I am (Zone 4-5 Canada), this year almost all my queens took a break. I opened them on the first 20 degree day we had and no brood in any carni hives, and a very small patch starting in a hybrid strain hive I have down in a valley in Zone 5 (I live right on the zone 4-5 line). Eerything else looked perfect - healthy population, nice clean cells toward the center, pollen and honey stores left over from winter, clustering up properly, etc…
As someone above said, this is very regional topic, with which I completely agree. Beekeepers from more southern locations will swear that if there is no brood, there is no queen. I have personally witnessed that to be false on several occasions. I’m still learning the patterns here for my area (only on year 4 for beekeeping), but here is what I’ve learned over a few years of my own experience from my 4 or 5 hives in my area.
1 - The first 20 degree day or two is critical for the queen to start (or significantly ramp up) brood production in my area. Where I live, this can be anywhere from March to May. This year was late, our first 20 degree day was this week. Last year was early, in March. I saw them starting to bring back pollen from who knows where the first week of April last year. This year, they just started bringing in pollen this week. Until that 20 degree day or two happens, and I allow about a week to pass, I don’t even bother looking for brood. In fact,openeing the hive this early I believe to be more detrimental when you can;t really change anything at this time anyway. Based on my experience, she takes a break somewhere in the winter until this time. There is generally not a single drone to be found, either.
2 - If they are bringing in pollen, but I have no brood in Spring, I still don’t worry. I have found that Carniolans especially can wait until the troops are bringing in pollen steadily and the temps have hit 20 degrees a few times BEFORE the queen even starts brood production. But in general, I have dealt with queenless hives, and it is very rare to see them bringing in pollen without a queen and/or brood. They simply have no need for it (unless a laying worker scenario perhaps). Steadily incoming pollen generally implies a healthy hive all the way from population to brood/queen health.
3 - When you first open the hive, choose a day that is cool but not too cool - maybe 15 or 16 degrees. Up here, the bees will still cluster up with the hive open at this temperature, generally around the queen, giving you a clue as to where she is.
4 - If they go queenless during the winter, there will almost certainly be a sign of them trying to recover - queen cells drawn, laying worker evidence, etc…
So I think in summary, I do not make the assumption that no brood = no queen until the weather has sufficiently warmed up. Even then, the colony needs to show me that there is no queen (not bringing in pollen, dispersed clustering, old queen cell evidence, etc…) before I act. Based on this post, my initial prognosis if this hive was in my area, would be that she is probably there, just on a break - until the OP found an old queen cell in a May, that is. 
