Most terror suspects are kept in Belmarsh jail, considered the UK's most secure.
It's so difficult to get in and out that lawyers complain about their own access to visit their clients.
They often cite the massive security checks as the reason for delays in court cases.
Jail breakouts are rare and no prisoner has ever escaped from Belmarsh, a category A jail in southeast London, though some have tried.
So why wasn't Daniel Abed Khalife, a suspect facing serious terror charges, banged up in modern Belmarsh?
Instead he was awaiting his trial in HMP Wandsworth, a category B jail in southwest London built 170 years ago and described in a watchdog report two years ago as "overcrowded, crumbling, vermin-infested" and suffering with staff shortages.
The same report said an inmate managed to escape from Wandsworth in 2019 and highlighted continuing concerns about security.
And don't forget that as a soldier, Khalife, 21, would have been trained to escape captivity of all sorts and take psychological advantage of less experienced captors.
That may go a long way to explain how he was able, as it's thought, to hide under a food delivery truck and hang on as it left the kitchen area where it's believed he was working as a chef.
"The issue is one of routine, coupled with prison staff shortages," said Mark Leech, editor of the Prisons Handbook for England and Wales.
"Perhaps the use of civilian caterers who are not trained prison officers, along with gate security procedures that just become routine and which he may well have spotted while going to court."
The public might be surprised to learn that a young terror suspect, accused of gathering details of his colleagues that could be useful to a terrorist and collecting information that could be useful to an enemy, had a reduced security risk rating - not A but B.
Mr Leech said: "That is something that in hindsight they will want to review and the investigation will look into that.
"He may well have given the impression to inexperienced staff who conducted his security categorisation that he was far less of a security escape risk than in reality he really was."
Police seem confident Khalife will be caught soon, but some Wandsworth escapees stay free for a very long time.
Great Train Robber Ronnie Biggs climbed over the wall with a rope ladder and jumped to a waiting removals van to flee Wandsworth in 1965.
He did return, but only by his own choice.
He wasn't arrested until his plane landed in the UK, 36 years later.