Mice and voles are a possibility, as are slugs and snails - I've had a few apparent "almost no shows" over the years which I am almost certain was due to slugs wiping them out as they emerged.
As for the viability of the seeds, peas are very easy to chit - soak them for a few hours, drain, and chuck into a plastic box with a piece of damp kitchen paper in the bottom, and cover. In a couple of days you should see little roots starting to appear - sow them as normal ASAP. I started doing that after the "no show" incident using saved seeds that I weren't sure had been collected and stored correctly, and almost 100% germinated.
Pre-chitting does at least give the assurance that the seed is viable and may make the difference between success and failure, especially if the soil conditions are unfavourable (i.e. very cold and wet.)
My pea-growing is necessarily on a rather small scale so chitting is no bother - you may well feel differently about it if you grow on an industrial scale like wot DD does!
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