This option is only available if you have a store, so the best thing to do would be to revise your auctions - on ones that don't currently have a bid, you would be able to put a big note at the top, and you can add to the description on those that do have bids (it will appear at the end of the listing, though, and I recommend making the text stand out in terms of size, to give people the best chance of seeing it).
Depending on how long you'll be away for, if you're going to be completely unreachable for several days (i.e. absolutely no access to internet), I would consider ending the auctions that don't have a bid.
