Regarding the set 75902 Scooby Doo Mystery Machine - it is a retired set according to the UK LEGO Shop at Home site:
The US LEGO Shop at Home site has not yet applied the "Retired Product" label, but shows this set as Sold Out:
As a general rule, it is best to check both US and EU Shop at Home sites (one of the many EU sites is enough) as set retirement information is maintained separately for both markets. Alternatively you can look at the Brickset page for a given set, it shows availability information for all markets (though it doesn't give a definite "Retired" status).
LEGO set retirements are rather unpredictable. On average, sets are available for around 2 years - but it greatly varies. Some sets have retired after 6 months on the market (76039 Ant-Man Final Battle is a recent example). Some sets stay available for more than 5 years (10220 Volkswagen T1 Camper Van is an example of this, and so was 10188 Death Star).
As you mention - getting Out of Stock at retailers (including LEGO Shop at Home) is often a sign of a set retiring, but it could also mean a temporary shortage of product. Finally, there are sets that are produced seasonally and those will go to Sold Out for longer periods of time at LEGO Shop at Home, only to return closer to the season - for example 10254 Winter Holiday Train.
Back to Scooby Doo - I do not think that LEGO has the Scooby Doo license anymore (though they could always opt to renew it at some point) and as such, most AFOLs consider this line to be done. As The LEGO Group is a private company, outsiders can typically only guess the plans - TLG could always surprise us.
And maybe some hope: Since the Mystery Machine set has not been retired for long, at least in the US, you might still be able to find one in places like Toys 'r Us or KMart, where they typically do not get rid of their stock very fast.
UPDATE: Not even 3 hours after posting this, the set has now gone to Retired Product status on the US Shop At Home site as well: