Welcome to the fold of the flying Triangle !!
Those buttons are only ‘lawyer proofing’ ie not just to show people the bars are fully engaged, but also to make the poor end user press them, everytime the bars are folded.
Early Mk3’s did not have them, they were only added later (I know cos I got one).
The buttons are spring loaded from below by a simple bent bit of flexible steel/brass, so if you can see the button, thro the hole, you should be able to nudge it back into the hole where it should pop up again. otherwise the bar can be removed by really loosening the QR lever screw under the stem, and jiggling it out as shown >> on page 8 >>http://www.strida.nl/english/images/Owner%20manual%20Strida%203.pdf as some Strida3’s had user fit folding bars.
Good news - The button holes in the outer stem forging that grip the bars, gradually wear over a few years use (or if accelerated by a file 
) … so that the buttons just become ‘visual indicators’ that the bars are ‘in’ with a slight ‘grip’ effect, but they do not need to be pressed, the fold is done just by pulling the bars out - much more user friendly and fast.