In The Picts and the Martyrs, Dick suggests that they go underground to avoid the Great Aunt, Like badgers. In lots of places people think they're extinct. But they aren't. Only they never let themselves be seen. Then he said If only the igloo wasn't on the other side of the lake, and Dorothy replied We could have lived in that alright. We'd be Picts .... Ancient Britons. Prehistorics. Original inhabitants. They had to hide from the invaders and went on living secretly in caves, and in the end people thought they were fairies and used to leave milk outside the door for them .... I heard Father talking about it (PM3).
Then Peggy says What about The Dog's Home and Nancy jumped off the ground and replied Well done, Peggy .... Dick .... Lets go and have a look at it at once, though Cook said That old place .... (PM3).
When Nancy says Badgers .... or Britons Dorothea says Picts (PM4). She tells the doctor that Dick and I are being Picts .... That's us (PM8). When Nancy addresses the note with the pigeons "Pict Post", Dot thinks She oughtn't .... Oh well, I suppose it doesn't matter (PM10).
Dorothea writes to Mother: Please ask father about Picts .... living secretly in caves and only coming out at night long after other people had conquered the land .... We'd both like to be sure (PM7). Mother writes back saying Your father says that in a way you are right and in a way you are wrong. There used to be a theory among folklorists that the origin of the belief in fairies and such was the half secret presence in remote places of the original inhabitants of the country who had been for the most part driven out by conquering tribes .... He says it is an exploded theory, but he also says most theories get exploded sooner or later (PM14).
They find that Jacky has been using The Dog's Home too; Dorothea says that he is a better Pict than either of us (PM11). She plans (PM14) to write her next story on a Pict, the very last of his race, living his life out to the end in a country whose people never even knew that he was there.
In Great Northern?, they find a turf-covered mound above the cove. Dorothea says that it is a real pre-historic Pict House like the one they saw on Skye. Dick measures it to tell their father. They find a box with the diary kept by the young chieftain, Ian McGinty, and Titty works out that it is in Gaelic (GN4).
Later Roger uses the Pict House as a look-out. Young Ian finds him asleep there and plays a trick on him. The old Pict-house is Ian’s private hiding-place and look-out ever since he had been a very small boy (GN16).
The Picts were a group of Late Iron Age and Early Medieval Celtic people living in eastern and northern Scotland: see Wikipedia:Picts