Hi Richard,
A more functional implementation of your solution.
lowerB = 99.434; upperB = 105.566;
MapIndexed[If[Between[#1, {lowerB, upperB}], {#2, #1}, Nothing] &, exampleMat, {2}] //
Flatten[#, 1] &
(*
{{{5, 37}, 104.}, {{5, 59}, 100.}, {{5, 181}, 100.}, {{5, 629}, 104.}, {{5, 785}, 100.},
{{9, 113}, 102.}, {{9, 295}, 102.}, {{9, 419}, 100.}, {{9, 439}, 102.},
{{12, 99}, 100.}, {{12, 303}, 100.}, {{12, 653}, 100.}, {{12, 707}, 100.},
{{12, 803}, 100.}}
*)