+++1,2,3 on them being jumpers. just my opinion, but i believe they can find that small opening very easily, especially at night when the surface silhouette is more defined. at the lfs i used to work in we had more fish jump out of tanks with small openings than we did with the ones that only had half of a glass lid. maybe just luck, but it held true for years there.
they do sometimes bury completely, but this is usually when being chased by a net or collection box. i have had a couple of pairs over the past decade and some are out in the open right away and some stay hidden for a while... all in the individual. also, they can repeat this behavior when moving or upgrading aquariums. just be patient and feed them plenty of meaty foods.
good luck with yours.