"Honey, my PIC ADC pin sagged my peak detector voltage!"

[HankMcSpank]

This is expected behaviour. Pullups will put 5V on the ADC input. Unless you meant to ask something else.....

That's just it - I never asked for or wanted pullups on the ADC pin, so was puzzled when 5V appeared there in the absence of a signal. The datasheet say WPUs are disabled at power up (& my code didn't enable them), so there shouldn't have been 5V on the ADC pin.....what I had to do in the end was disable them myself in my code - even though they should be disabled by default.

In my opinion there's a problem with the PIC16f1828's startup settings vs what Microchip say in their associated datasheet.