Some people collect stamps or coins or old photographs. Methos collects secrets hidden away and hoarded, some to be shared out and some to be kept, some so old they are no longer secret.
Secrets which Methos once kept that are now common knowledge:
Smoking is bad for your health
The Pharaohs were only human
The men and women executed for witchcraft, were (with a few exceptions) innocent people murdered by the ignorant, the fearful and the greedy.
The Earth is round....
and goes around the sun
Methos can not really remember where he learnt the last two but they have been bits of information, he has always known, much like one does not remember learning that Paris is the capital of France or fire is hot.
Secrets Methos now no longer remembers:
He's a lot older than 5000 years
Where immortals come from
The location of the lost city of Atlantis
His original name
Secrets Methos has given up trying to tell people:
There will never be only one. The story of the gathering is a corruption of an old Sumerian myth.
There is no good, there is no evil only people, and societies. - A thousand years from now people will decide that using fossil fuels, or drinking alcohol, or maybe using paper was evil and a new set of young immortals will decide his actions in the current century were beyond redemption.
Everybody dies eventually – even immortals
There was once seventeen planets in the solar system, now there's only twelve (whatever 'science' may say on the subject)
Life on Earth was originally 'seeded' by an ancient alien race
Experience quickly taught him the wisdom of keeping any knowledge counter to current popular opinion to himself, people generally believe what they want to.
The secrets he intends to tell Duncan include:
He puts the money to pay for his drinks in the till with interest after Joe closes up (They enjoy winding Mac up too much to let him in on the secret yet)
Amanda and Methos have regular bets on who can manipulate Duncan into a particular action first.
Methos may have liked Silas, but Kronos was his brother and he loved him right up until the day he died. He doesn't regret his part in their deaths though.
When Methos offered Mac his head he knew he wouldn't take it.
Methos has had 68 wives; and 12 husbands.
There are some secrets that Methos will never tell Duncan Macleod, including:
Methos killed on holy ground once, nothing happened except the usual struck by lightening. He will never tell Duncan because he likes the idea of sanctuary too much to ever risk other immortals discovering the truth.
There's really no such thing as a dark quickening or a holy spring, the subconscious is a powerful thing.
He once spent nearly a century pretending to be a woman, only three people ever noticed he wasn't, two of them were children the other one was insane.
Methos can identify a person by their quickening, he may trust Mac more than anyone else but he still believes one should never give up your edge.
Methos gave Darius the Watcher's chronicle he had when he died
Methos still doesn't believe in Ahriman (or alternate worlds)
Methos would sacrifice his own life to save Macleod