It was not, but reaction was not adequate
First of all, it should be mentioned that both Germans and Romanians recognized weakness of Romanian armies on flanks of Army Group B already in late September, early October of 1942. This happened practically immediately as Germans were pulling away their divisions from the flanks in order to employ them in the city fighting. Antonescu himself told German representative (while they were discussing creation of purely Romanian army group on October 5th) that he fears of Soviet offensive in sector of his troops. Reinhard Gehlen also noted the possibility, although he assumed that main Soviet effort would be near Rzhev, as Soviets were visibly massing for another offensive (Operation Mars).
Somewhere mid-October German reconnaissance started discovering build-up of Soviet forces between Don and Volga. At this point Germans were still convinced this would be limited offensive, nevertheless they finally authorized long promised artillery reinforcements for Romanians. 22nd Panzer
division was stationed behind 3rd Romanian Army and together with 1st Romanian armored division supposed to be mobile reserve to contain any Soviet breakthrough. Unfortunately for the Axis, both divisions were partially armed with obsolete Czech Pz35(t) and Pz38(t) tanks, and were not at full strength.
In early November, even Hitler noted weakness of German flanks in Stalingrad and ordered 6th Panzer Division to be moved from France to area of Army Group B. This division was rested and refitted with modern tanks, but will arrive after Uranus already trapped German forces in Stalingrad and will be used in relief efforts. It should be noted that transferring whole division from France or Germany to Eastern front usually took few weeks or even whole month. At the same time, Hitler ordered training programs for Romanian troops in anti-tank combat. But again, this was too late, as this training would commence only in spring of 1943.
Few days before the Uranus started Germans were well aware that something would happen. In fact, they have planned to immediately withdraw all motorized and panzer forces remaining in the city (14th and 24th Panzer divisions above all ) to act as a reserve, as soon as their last offensive to fully capture it (Operation Hubertus) was finished. At this point Soviet holdouts on the right bank of Volga were practically few meters from the water and Germans were finally close to long awaited victory.
Why were measures inadequate ? Situation for Germans was getting from bad to worse that November, with reinforcements needed everywhere. First, they lost at el Alamein and Rommel was in headlong retreat. Second, Operation Torch started and few available reinforcements had to be shipped to Tunisia, most notably 10th Panzer Division. In connection with that, 2nd SS Panzer corps was used to capture remaining territory of Vichy France so its formation and refitting was slowed down, therefore being able to transfer to Eastern Front only in early 1943 to participate in Third Battle of Kharkov. In sector of Army Group A, Germans were still trying to advance in late October and early November, but were being counter-attacked and could not spare troops for Army Group B. Finally, while Soviets did lose bloodily in Operation Mars, this attack did tie down lots of German units in the center part of the front.
Overall German assessment (and perhaps wishful thinking) was that after heavy losses in 1941 and 1942, Red Army is on its last legs and could mount only limited offensive in the South, and perhaps main effort again around Rzhev. While they have notices Soviet buildup between Don and Volga, and in Don bridgeheads that Red Army retained, they did not expect Soviet goal to be drive to Kalach and complete encirclement of 6th and parts of 4th Panzer army. Instead, they assumed more modest goals like relief of Soviet troops in Stalingrad or simply expanding bridgeheads at Don.