That is indeed strange, Alan.
Ordinarily, I don't subscribe to 'drumming up trade' theories. If any business wishes to remain in business, it is prudent not to alienate customers, and there is also the small matter of Trading Standards to consider.
Brake fluid is naturally hygroscopic and a biennial change is generally recommended. Having said that, and given that it was (allegedly) changed only 18 months ago and your nipples appear to not have recently been undone, I am inclined to agree with your conclusion (a).
Given that you have used the local garage for some years and presumably therefore have some degree of confidence in both their integrity and technical ability, I feel that your conclusion (b) is much less likely to be the case.
It may, of course, be much less sinister, and your brake fluid has indeed absorbed an excess of moisture, but both the time scale and the fact that only the master cylinder was affected weigh against that. The only other thing that I can think of is condensation due to rapid and extreme changes in temperature, but that is unlikely in the extreme.
If you would like a second opinion, I can recommend Carnetix in Melton Mowbray, if it is of any assistance. No connection other than as that of a satisfied customer, but please do mention my name if you should contact them.
Regards, John.
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Time flies like an arrow, fruit flies like a banana .....
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