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I would never fail to check and check recheck then check and check oil levels for that long, especially after such a major repair (timing belt or a seal etc)

Well, still, 2.5liters (almost 3 quarts) is a huge amount of oil.

But this oil didn`t get lost (or eaten up), at least not overnight.

I would keep an eye out all the time and watch the oil level strictly on a regular basis with every 50miles driven. If you do have an existing leak then that`s the way to go. Or, fixing the source of the problem, if there is any.

I would suggest fully completely clean the entire engine as step #1 and try maintaining the cleanliness of the engine, at all times.

this can be your best friend to be able to spot it (in time) when something goes wrong.