Yes a rat can have heart problems without showing any signs.
My rats have not shown symptoms of heart disease until the disease was very advanced.
Heart problems seem to be very common in rats. It seems to be more prevalent in older rats but I have had young rats with advanced heart disease as well.
I did have one rat, Caleb, whose heart problem became very serious in early 2005 after he, as a seemingly healthy young boy, underwent nebulizing as a preventative health proceedure. We assume the stress did it. Unfortunately Caleb was the first rat I had with a heart problem and it took 7 - 8 months of serious illness and repeated near death eposiodes before his heart problem was discovered.
An enlarged heart can be seen on an xray but other heart problems can only be detected by symptoms.
The symptom my rats have always displayed is a respir problem that does not respond to the various combos of antibiotics. Turning blue, and congestion in the lungs are two other symptoms that may indicate a heart problem. Enalapril, an ACE inhibitor, is given as a diagnostic tool and if it helps then a heart problem is assumed and other heart meds are added.
Rhubarb was put on heart meds recently. He had a very slight respir problem which was being treated by antibiotics. It was just by luck that we discovered that his heart is hugely enlarged as his symptoms are so slight I wouldn't have thought heart problem for a long time. My vet was xraying two other rats and since he does 3 at a time for the same price he decided to xray Rhubarb.
There was no way to know that your boy had an underlying heart condition.
I am so sorry.
I hope this info. helps.