Also if she has been there for the past 70 years then she was there during the 70s/80s/etc. so you have all sorts of possibilities:
Gomez sneaking out the vault to visit a rather confused Delgado in prison
Heck it’s the perfect opportunity for a Delgado/Gomez team up, with Twelve and Three exasperatedly teaming up to stop them. That’s my dream story right there.
Twelve and the Master sitting in the vault having tea together in 1982, and Twelve casually mentioning, “Oh, by the way, you’re going to steal a Concorde today,” and the two of them laughing about it.
During their New Year’s Eve party in 1999 (which Nardole is not invited to but he insists on supervising anyway), there’s a tremor which knocks over the punch bowl. “Sorry, my fault,” says Missy. “I’m trying to destroy the Earth right now.” “Don’t worry about it,” assures the Doctor. “I’m sure there’s a handsome, rather forgetful space genius out there who’ll fix things.” Nardole just sighs and folds his arms disapprovingly.
The Doctor quickly ducking into the vault during The End of Time and grumbling to the Master that he was in the middle of a really good lecture when all his students started to transform into Simm.
And of course there are all the times the Earth was invaded and it wasn’t the Master’s fault, and the two of them sit in the vault eating popcorn while they watch the news (because the Master insisted ages ago on having a TV of course), and Missy is like, “You do remember saving the day when this happened, don’t you?” and the Doctor replies, “I think so… or maybe that was a different time… After awhile all invasions start to look the same. I’m 90% sure I didn’t let the Earth get destroyed. Probably. Guess we’ll find out.”
