if you have a spectator enter to spectate an entity, and the entity moves (in this case via tp, not tested in other ways), if you try to:
(x1/y1/z1 = the pos where you started spectating, x2/y2/z2 = the pos where you are now)

/execute @a[x1,y1,z1,r=2,m=3] ~ ~ ~ /say oldpos
/execute @a[x2,y2,z2,r=2,m=3] ~ ~ ~ /say newpos

if you put that on a command block:

• it will always output [name] oldpos

if you type that on chat:

• it will always output [name] newpos