Elly, kittens often like to hide underneath cars (climb inside engines and play).
The worst part is we will never know. I have experienced and read of many cases where after driving a certain distance, kittens simply jump out in the middle of highways, because the engine gets too hot for them to stand.
Just a possible scenario.
If you suspect ada orang yang amik, try posting fliers around your housing area. Tengok kalau ada siapa2 yang call. Maybe they wandered far and someone took them in thinking they were strays.
Also, please keep your cats indoors yes. Kalau nak bagi kittens2 keluar main, please supervise, watch them. Then bring them back in. Also, kalau boleh, spay/mandulkan the mother cat
too many homeless kittens out there on the streets. instead of breeding more, it will be a nicer decision to adopt and give a stray a good home
Just my humble advice. I wish you luck with finding your kittens. Take care.