That is generally not the case. Unless Eucharistic Adoration is going on - and the Church must not be unattended in such an event - it is extremely likely that a church may be unlocked and unattended. Perhaps the increase in these desecrations points to new realities, and volunteers - gathered largely from the ranks of retirees with the time to do this - might be needed to "watch and pray" in the churches in order to keep them open all day, every day.
For many years, it has been true that most inner-city churches have kept their doors closed when no Mass or other scheduled event is going on. That's an awful shame; when my own parents were young here in Boston, virtually every Catholic Church was open 24/7. Except for a few shrines where there is a more-or-less guaranteed, continuous flow of people coming in and out all day, this is no longer the case. Even the cathedral is shut tight except when Mass is going on! It will be even worse if Catholic Churches "everywhere" have to bolt the doors, too.
It is a miracle of forbearance that Our Lord stays His Hand!
I am not sure if my Church locks the door at night. Not long ago, one of the Priests mentioned either it had started or was going to start.