I’ve often seen deacons without dalmatics, but the only times I’ve seen mass with celebrant sans chasuble were when hiking. Fr. Dan said mass in alb and stole, and it was the old military style travel stole at that, and had dispensation from the bishop to do so when hiking. (And explained that to us who were hiking with him before we left!) He also had an antimension* for use as a portable altar.
The hikes were also retreats.
Now I’ve seen several services done in alb and stole, without chasuble, but they were not the Mass.
But also, be aware that some of the other Roman Church useages besides the Roman Mass have other vestment rubrics.
Also, be aware that those vested but not concelebrating may vest “in choir” in Cassock and Surplice, or in Alb and Stole. They may perform some liturgical functions, even, when so vested. MHC, Master of Ceremonies, distributing of ashes, blessing of throats…
The Dominicans often had priests in choir assist with communion; they vested in alb and stole, or even habit and stole, but did not come to the altar during the consecration when so vested; they stood by the foot of the dais, and waited until just before communion to join the celebrant at the altar, and were handed the sacred vessels by the celebrant.
Also, the Syriac Rite Catholic Churches may use a cope or a chasuble. The latter is a latinization. Several of the Syriac churches refer to their divine worship service as the Mass when speaking in English, including the Chaldean and Maronite Catholic Churches. I doubt it is what the OP was encountering, but being aware of it may reduce future confusion as these churches de-laitinize and also become more evident due to media and internet presence.
- the Antimension is a corporal with relics sewn in, and signed authorization from the bishop to be used as an altar, usually written directly upon it. It is the norm for Byzantine use; for Roman Church use, it requires a dispensation and is only used for “good cause”… mostly by military chaplains. When used for the Latin use, another corporal is used on top of it, as if it were the altar stone itself.