I'm posting this in case it helps someone else as I can't see this information anywhere else online. We didn't see any left-luggage facilities in Albrook Airport, which is tiny.
Apparently there is a luggage storage facility in Albrook mall which charges $3 per day per item, but we didn't actually use this.
Instead we left our bags at the Albrook Inn, a hotel about 1km from the airport. Because we're staying there for one night the luggage storage was free, but apparently they charge a flat rate of $10 if you're not staying there.
Postscript: we returned after 2 weeks to find that all of our cloth suitcases were covered in mould, and their contents too, since the hotel left them in a non-ventilated room in the tropical heat. Our hard-sided plastic suitcases were OK. You may have better luck using a specialised storage facility.